1. Field of the Invention
The present invention relates generally to the compression and retrieval of data representing a font or other set of symbols; and, more particularly, to a method and apparatus for storing a large font, such as a Chinese or Japanese character set, in a compressed form while retaining access to individual symbols of the font.
2. Description of the Prior Art
In order to display messages in languages such as Chinese or Japanese on a CRT or an LCD display, a large set of symbols, or glyphs, is required. For example, the Chinese Unicode standard character set contains about 21,000 different Chinese symbols. Furthermore, each symbol is the size of at least some hundreds of pixels; and, as a result, to store a complete Chinese font requires a large amount of memory. Being able to store the glyphs in a more compact format than pure bitmaps will substantially reduce memory requirements.
For laser printers or high resolution displays, a font is usually stored as a series of points that are joined by curves. This brings the additional advantage of making font scaling possible, although for fonts stored in this way, some processing is needed to render the image itself. For lower resolution displays, font scaling is not of interest, and it would be more efficient to store the font as a bitmap.
The majority of lossless data compression methods known in the art work in a sequential manner by referring back to data that has already been encoded. Such methods are inappropriate for font compression, however; where, ideally, only a single glyph should be decompressed at a time. If sequential methods of this type are employed, some blocking of the glyphs is required, and a trade-off must be made between the two extremes of compressing the entire font as one block, thus losing random access capability, and compressing each symbol separately, in which case overall performance becomes quite poor.
Instead of the above-mentioned sequential codes, a two-part code can also be used to compress and retrieve a font. Typically, the first part of such a code describes the statistical properties, or a model, of the data; and the second part encodes the data by a code derived from the model.
Exemplary of font compression and retrieval methods known in the prior art include those described in U.S. Pat. Nos. 5,488,365; 5,058,187; 5,587,725; 5;473;704; and 5,020,121; and PCT Publication No. WO 98/16902. In general, however, none of these prior art methods describes a font compression and retrieval technique that provides complete random access of individual symbols of the font, which is important to permit high-speed access of the symbols by modern high-speed equipment.